This invention relates to a device for obtaining a control signal corresponding to the density of a fibre web lying on the cylinder of a carding machine.
The production of fibre slivers having cross-sections which are as uniform as possible represents a pressing problem in the textile industry. In particular, the fibre material should be measured with respect to its cross-section at an early stage where regulators for correcting undesirable deviations can be inserted using relatively simple means.
Various devices for measuring the cross-section of card slivers are already known and used. In order to allow the cross-section of the sliver and the sliver count to be regulated with as little delay as possible, there have been attempts to determine the amount of fibre at a location which is as near as possible to the position where the fibre material is metered in. On a card having a take-in regulator, it is the feed roller which simultaneously forms the regulating position. The feed roller charges the cylinder with fibre material via a licker-in. It is very convenient to measure, by means of the feed roller, the fibre covering of the cylinder from the viewpoint of determining the momentary fibre feed with as little delay as possible.
However, because of the structural design of a card there are only a few possible positions for a measuring device. It is particularly aggravating that the fibre material on the cylinder is finely distributed over its entire width, that the cylinder rotates at a relatively high speed, that its circumferential speed is high and that its surface is rough. The arrangement of a measuring device is also complicated by the fact that the cylinder is a very close fit in a housing and the aerodynamic conditions between cylinder surface and housing should not be disturbed. Up until now, all of these aggravating circumstances have made it impossible to provide a reliable measuring device which is both insensitive to the rough operating conditions and suitable for the requirements of regulation.
Optical measuring devices are known but are not suitable for protecting the entire width of the fibre covering and are not, therefore, suitable as measuring devices for regulating the cross-section of the sliver.
It is also known to make use of a pneumatic measuring device which utilizes the pressure differences in the cylinder housing caused by the fibre covering of the cylinder. However, the device needs several measuring positions on the circumference of the cylinder and is not suitable for measurement on the licker-in, the doffer or the take-off rollers.